SONET rings have become the standard for providing fast restoration capabilities in the high-capacity transport networks. Robust operation and millisecond restoration times are achieved in SONET rings by having a single predefined restoration line, that being the protection lines around the side of the ring opposite the failure. One disadvantage, however, of a single restoration line is that traffic is vulnerable to secondary failures that take down or preempt the restoration line.
Analysis has shown that secondary failures are rare on any given ring. Because of this, even if the recovery from secondary failures is assumed to require physical repair that takes hours to complete, the contribution to average unavailability of circuits routed over a given SONET ring is negligible. However, given the large quantities of SONET rings being deployed in major carrier networks, the probability of secondary failures causing ring traffic to be dropped somewhere in the network becomes appreciable. Further, customer perceptions of reliability are often based more on memory of a single extended outage rather than on long term average performance. Thus, there is potential value in having the ability to quickly recover traffic dropped because of secondary failures on rings and thereby avoid embarrassing extended outages.
Digital cross-connected automated restoration schemes, which also provide primary restoration for mesh facilities, have been proposed as a means for recovering failed ring traffic as a result of secondary failures. This scheme is particularly attractive given the trend toward integration of add/drop multiplexer capabilities into digital cross-connects. The basic approach of these schemes is to use a centralized processor to collect alarms, to survey the state of failed facilities and then find alternate routes for the failed traffic over spare or protection capacity elsewhere in the network. The centralized processor instructs the digital cross-connects to reroute the traffic.
A significant disadvantage of dynamic restoration schemes is that proprietary systems must be developed to collect alarms and facility information, plan restoration capacity, calculate alternate routes and issue cross-connect commands. The cost associated with this development would be particularly difficult to justify for a ring-only network where the systems only support recovery of rare secondary failures.
Another issue with a back-up scheme that involves finding spare capacity in real time is that perceived benefit of sharing protection capacity with other rings may not be realized in a sparsely connected network. Consider, for example, an office with only two fiber routes out of the office, with two or more rings having terminal nodes in that office. A likely cause of a secondary failure would be a fiber cut, which would effect all the rings, and then a circuit pack failure that would effect a single ring. The traffic dropped from the ring with the secondary failure could not be recovered using protection capacity on the other co-terminous rings because the protection capacity of the other rings would be in use for ring switching in response to the fiber cut. Thus, the only way to recover the dropped traffic using the other co-terminous rings would be to use service capacity on the other rings. Planning, administration and monitoring dedicated service capacity across a ring network to protect against the variety of possible second order failures would be complex and costly.